1. Field of the Invention
The present invention relates to a toner to be used in an image-forming method or toner jet method for visualizing an electrostatic image in electrophotography.
2. Description of the Related Art
An electrophotographic method involves: charging an electrostatic latent image bearing member composed of a photoconductive substance by various means; exposing the charged electrostatic latent image bearing member to light to form an electrostatic latent image on the surface of the electrostatic latent image bearing member; developing the electrostatic latent image with toner to form a toner image; transferring the toner image onto a transfer material such as paper; and fixing the toner image on the transfer material with one or both of heat and pressure to provide a copied article or print.
When such image-forming process is repeated a large number of times, ozone produced in the charging step of charging the electrostatic latent image bearing member reacts with nitrogen in the air to form a nitrogen oxide (Nox), the so-called corona product. Further, such nitrogen oxide reacts with moisture in the air to form nitric acid, which adheres to the surface of the electrostatic latent image bearing member. As a result, the resistance of the surface of the electrostatic latent image bearing member reduces, and, at the time of image formation, image smearing serving as an image defect on the electrostatic latent image bearing member occurs; the phenomenon is particularly remarkable under a high-humidity environment.
By the way, nowadays, an output apparatus employing electrophotography, which has been conventionally used for personal-use purposes, begins to be used for professional-use purposes; in particular, the apparatus has started to be put into full-fledged use in light printing applications (print-on-demand (POD) applications where a large number of kinds can be printed in small amounts each).
When it is assumed that an electrostatic latent image bearing member is used in a light printing application where high reliability and high durability are requested, an improvement in durability of the electrostatic latent image bearing member requires an increase in surface hardness of the member.
However, when the durability of the electrostatic latent image bearing member is improved as described above so that the electrostatic latent image bearing member can be used in a POD application, the surface of the electrostatic latent image bearing member is hardly shaved and hardly refreshed, so the improvement is disadvantageous for the prevention of image smearing.
In view of the foregoing, the following procedure has been conventionally adopted: a heater for temperature regulation (hereinafter referred to as “drum heater”) is provided for the inside of the electrostatic latent image bearing member so that the problem known as “image smearing” is dissolved. However, it has been demanded to perform the removal of the drum heater for energy-saving purposes because the drum heater requires electrification and consumes power even at nighttime during which the drum heater is not used.
In addition, even when such a constitution that corona products on the electrostatic latent image bearing member are removed with an air blow or the like is established, an increase in amount of the corona products to be produced precludes sufficient removal of the products, so it may become impossible to inhibit the occurrence of an image defect with reliability. The frequency at which corona products are produced increases particularly in a high-speed developing system to be used in a POD application, so image smearing is apt to be induced in the system.
To cope with the image smearing, the following approach on a toner side has been conventionally adopted (Japanese Patent Application Laid-Open No. Hei 5-273785): an inorganic fine powder is added to toner, and the image smearing is prevented by the abrasive action of the powder. However, when the approach is employed in the above-mentioned POD application, an electrostatic latent image bearing member is hardly abraded, with the result that corona products cannot be sufficiently removed. In addition, when fine particles each of which can serve as a spacer are added to the toner, a phenomenon in which the spacer particles are liberated occurs, so image defects due to the contamination of the inside of the apparatus and insufficient charging of the member are prompted in some cases.
In view of the foregoing, the following attempt has been proposed: corona products are removed by utilizing a substituting ability which a laminar compound and a porous material each have instead of the abrasive action. For example, Japanese Patent Application Laid-Open No. 2003-066637 proposes a method involving feeding a compound having an acid-receiving effect such as a hydrotalcite compound. The hydrotalcite compound is a laminar compound composed of a positively charged layer and a negatively charged layer. The compound reduces influences of the corona products by the following action: CO32− in the structure of the compound is ion exchangeable, so CO32− adsorbs an acid by easily substituting with any other anion. However, it is hard to say that the compound has a sufficient adsorbing ability because the sites where the ions can substitute are mainly the edge portions of the respective layers.
In view of the foregoing, it has been proposed that zeolite or the like be used as a polar adsorbent (Japanese Patent Application Laid-Open No. 2003-91223). A porous material such as zeolite has a larger number of substitution sites and a higher substituting ability than those of a compound of a laminar structure. However, zeolite as a porous material has a pore diameter as small as about 1.5 nm, and a wall between a pore and an adjacent pore is thin, so zeolite has a low physical strength as a porous material. Since toner is stirred at a high speed particularly in a developing device to be used in a POD application, the following detrimental effect arises: the shapes of porous external additive particles are broken by a sliding action between the particles, and the broken external additive particles affect the parent body of the toner.
It has been proposed that a porous powder impregnated with silicone oil be used as a porous material excellent in stability against a mechanical stress (Japanese Patent Application Laid-Open No. Hei 2-221964). However, the powder is impregnated with silicone oil so that the pores of the powder are filled. As a result, there is a possibility that the pores cannot sufficiently exert their substituting abilities, and corona products cannot be sufficiently removed. In addition, the particles of the powder each have a large particle diameter; specifically, the particles each have a primary particle diameter of from 2 μm or more and to 15 μm or less. As a result, the powder serves as a liberated external additive particularly in a high-speed developing system to be used in a POD application, and the scattering of the powder may lead to the contamination of the inside of the system, or may cause the destabilization of the charging of the system.
Further, it has been proposed that the developing performance of toner be improved by adding a calcium phosphate compound having a porous structure (Japanese Patent Application Laid-Open No. 2002-287414). However, the compound used has a large pore diameter, and cannot sufficiently capture corona products, so the proposal is not a sufficient measure against image smearing.
As described above, at present, no toner capable of achieving compatibility between the removal of a corona product and a combination of high durability and high stability in a high-speed developing system for a POD application using no drum heater is still available.